The expression of the IE genes of herpes simplex virus is controlled by the assembly of a multiprotein enhancer complex that requires the interaction of both viral (aTIF) and cellular (Oct-1, C1) transcription factors. Previously, isolation and expression of cDNA clones encoding the mammalian C1 factor have indicated that this protein is a novel transcription factor. As interactions of this protein with other cellular components would yield insight into the significance of the C1 protein, genetic screens were designed to isolate additional cellular factors which interact with various domains of the protein. These screens have resulted in the isolation of cDNAs encoding a number of characterized and novel polypeptides. Of these, the isolation of a subunit of RNAPII has indicated that the C1 protein may activate transcription via unique, direct interactions with the mammalian polymerase. Future studies are designed to analyze this interaction as well as to characterize additional cDNAs which may encode proteins which play significant roles in the regulation of cellular and viral gene expression. One of the more striking characteristics of the C1 factor is the presence of 20 amino acid reiterations within the central domain of the protein. These related sequences represent sites at which the 220 kD precursor is specifically proteolytically processed to generate a family of related polypeptides. However, as the cleaved products were shown to remain tightly associated as a multisubunit factor, the role of the processing remains unclear. To understand the biological significance of this processing relative to the activity of the C1 factor and to isolate the specific protease responsible for this processing, a novel genetic screen has been developed. Presently, the assay focuses upon two model viral protease systems which will allow for the optimization of general conditions for the isolation and characterization of novel proteases and their substrates. In this manner, the system may also provide a unique tool for antiviral drug development.